VAM is an important monomer building block in the synthesis of plastic polymers. The main fields of use of VAM are i.a. the production of polyvinyl acetate, polyvinyl alcohol and polyvinyl acetal and also co- and terpolymerization with other monomers such as for example ethylene, vinyl chloride, acrylate, maleinate, fumarate and vinyl laurate.
VAM is produced predominantly in the gas phase from acetic acid and ethylene by reaction with oxygen, wherein the catalysts used for this synthesis preferably contain Pd and Au as active metals and also an alkali metal component as promoter, preferably potassium in the form of the acetate. In the Pd/Au system of these catalysts, the active metals Pd and Au are probably not present in the form of metal particles of the respective pure metal, but rather in the form of Pd/Au-alloy particles of possibly different composition, although the presence of unalloyed particles cannot be ruled out. As an alternative to Au, for example Cd or Ba can also be used as second active metal component.
Currently, VAM is predominantly produced by means of so-called shell catalysts in which the catalytic active metals of the catalyst do not fully penetrate the catalyst support formed as a shaped body, but rather are contained only in an outer area (shell) of greater or lesser width of the catalyst support shaped body (cf. on this EP 565 952 A1, EP 634 214 A1, EP 634 209 A1 and EP 634 208 A1), while the areas of the support lying further inside are almost free of active metals. With the help of shell catalysts, a more selective reaction control is possible in many cases than with catalysts in which the supports are impregnated into the core of the support with the active components (“impregnated through”).
The shell catalysts known in the state of the art for producing VAM can be for example catalyst supports based on silicon oxide, aluminium oxide, aluminosilicate, titanium oxide or zirconium oxide (cf. on this EP 839 793 A1, WO 1998/018553 A1, WO 2000/058008 A1 and WO 2005/061107 A1). Catalyst supports based on titanium oxide or zirconium oxide are currently scarcely used, however, since these catalyst supports display no long-term resistance to acetic acid and are relatively expensive.